1. Field of the Invention
The present invention relates generally to a double side printing apparatus for printing on opposite sides of a recording medium and, more particularly, to a double side printing apparatus for making one-side printing feasible.
2. Related Background Art
A printing apparatus has been widely utilized as an output apparatus of a computer. An electrophotographic apparatus capable of printing on an ordinary sheet of paper has been utilized as the printing apparatus. In response to a demand for saving natural resources in recent years, a double side printing apparatus for printing on opposite sides of the sheet has been required. Then, an apparatus provided with both a mechanism for printing on the right side of the recording medium and a mechanism for a printing mechanism for printing on the reverse side of the recording medium, is required for increasing a printing speed.
FIG. 12 is an explanatory view showing the prior art.
The perfecting (double side printing) apparatus includes a printing unit 9b for printing on the right side of a sheet of recording paper P(which is hereafter simply referred to as the sheet P), and a printing unit 9a for printing on the reverse side of the sheet P. The sheet P is classified as a continuous sheet perforated per page. The printing unit 9a for printing on the reverse side has a photosensitive drum 90. The photosensitive drum 90 is charged by an unillustrated pre-charger and thereafter exposed to a light image by an exposing unit (not shown). An electrostatic latent image corresponding to the light image is formed on the photosensitive drum 90. The latent image on the photosensitive drum 90 is developed by an unillustrated developing unit. Then, the developed image on the photosensitive drum 90 is transferred onto the sheet P by a transferring unit 91. Thus, the image is printed on the reverse side of the sheet P.
The printing on the right side of the sheet P is likewise performed. To be specific, the printing unit 9b for printing on the right side has a photosensitive drum 92. The photosensitive drum 92 is charged by an unillustrated pre-charger and thereafter exposed to a light image by the exposing unit (not shown). An electrostatic latent image corresponding to the light image is formed on the photosensitive drum 92. The latent image on the photosensitive drum 92 is developed by the unillustrated developing unit. Subsequently, the developed image on the photosensitive drum 92 is transferred onto the sheet P by a transferring unit 93. Thus, the image is printed on the right side of the sheet P.
Toner images are fixed onto both sides of the sheet P by a fixing unit 94. The thus constructed perfecting apparatus for printing on the continuous sheet is disclosed in Japanese Patent Application Laid-Open Publication Nos.7-77851 and 8-211664.
There arise, however, the following problems inherent in the prior art perfecting apparatus.
With a diversification of printing modes, there is a mode for printing on only one side of the recording sheet. With this demand, the perfecting apparatus is required to print on only one side of the recording sheet. In this case, a problem is that the prior art perfecting apparatus does not have a one-side printing mode, and hence the one-side printing is difficult to perform.